Austenitic stainless steel

ABSTRACT

Embodiments of an austenitic stainless steel include, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; greater than 1.5 to 4.0 manganese; 0.20 to 0.75 niobium; up to 0.1 titanium; iron; and incidental impurities. Certain other embodiments of an austenitic stainless steel include, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; up 4.0 manganese; 0.20 to 0.75 niobium; at least one of no greater than 0.1 titanium and no greater than 0.1 aluminum; iron; and incidental impurities.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) from U.S. provisional patent application Ser. No. 60/687,400, filed Jun. 3, 2005.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This work, in part, was funded under United States Department of Energy contract no. DE-FC02-OOCH11062.

BACKGROUND OF THE TECHNOLOGY

1. Field of Technology

The present disclosure relates to austenitic stainless steels. More specifically, the present disclosure relates to austenitic stainless steels having improved creep resistance and/or improved corrosion resistance when subjected to high temperature environments.

2. Description of the Background of the Technology

High temperature air presents a particularly corrosive environment. Even more aggressive corrosion conditions can occur if significant water vapor is present. The combination of high temperature air and significant water vapor is common in energy generation devices such as, for example, gas turbines, steam turbines, and fuel cells, and in heat exchangers and recuperators handling the gas streams used or generated by such energy generation devices, as well as in equipment for treating, processing, or extracting chemicals or minerals at high temperatures. Accordingly, parts of such devices subjected to these conditions have been fabricated from a variety of austenitic stainless steels.

To enhance corrosion resistance, austenitic stainless steels include various combinations of chromium, nickel, manganese, and other alloying additions. Nevertheless, stainless steels and certain other chromium-bearing heat-resistant alloys are susceptible to attack in high temperature air and in high temperature air containing water vapor. This attack takes two distinct forms. Low-alloy content stainless steels such as, for example, AISI Type 304 (nominally 18 weight percent chromium and 8 weight percent nickel, balance iron), suffer from accelerated oxidation in the presence of water vapor. The slow-growing chromium oxide film is displaced by a thick scale comprised of rapidly growing mixed iron and chromium oxides. The result is rapid metal wastage by conversion to oxide. High-alloy content materials such as, for example, superferritic iron-chromium stainless steels and nickel-chromium superalloys, appear to be immune to this form of attack, but have been observed to suffer from weight loss during exposure to water vapor. The oxide that forms on certain of the high-alloy content materials is very pure chromium oxide and is susceptible to evaporation through the formation of volatile chromium oxyhydroxides. The result of this evaporative loss of chromium to the atmosphere is an abnormally high level of chromium depletion in the metal substrate, and this can lead to a loss of high temperature oxidation resistance. The transition between the foregoing corrosion states is relatively complex, with aspects of both states noted in some alloys.

In addition to corrosion, articles and parts in high temperature environments may suffer from creep. Creep is the undesirable plastic deformation of alloys held for long periods of time at stresses lower than the normal yield strength. Thus, creep may affect certain structural parts and other parts subject to high stresses and high temperatures in, for example, energy generation devices and related devices, and in equipment and parts for high temperature processing, treating, or extracting chemicals or minerals, or for high temperature treating or processing alloys. In such applications, it is often desirable that parts are formed from a material that has substantial resistance to corrosion in high temperature environments, and that also has substantial creep resistance.

The alloying element manganese has been shown play a role in mitigating the effects of chromium oxide vaporization. Many stainless steel specifications include manganese at levels limited to 2 weight percent or less, with no required minimum level. The manganese in these steels is not an intentional alloying addition but, instead, is included in the steel as an incidental ingredient derived from the scrap starting materials. One austenitic stainless steel adapted for use in high temperature, high water vapor content environments that includes an appreciable allowance for incidental manganese is NF709 alloy. NF709 alloy has been available from Nippon Steel Corporation in forms including of seamless tubing for boiler applications. The composition of NF709 alloy, which is provided in the Nippon Steel publication “Quality and Properties of NF709 Austenitic Stainless Steel for Boiler Tubing Applications”, is shown in Table 1. The published composition specifies a manganese limit of 1.5 weight percent, with no specified minimum. According to various published accounts of research on this alloy, the typical commercial manganese content is approximately 1 weight percent. Certain other austenitic stainless steels are also shown in Table 1. Elemental concentrations throughout the present description are weight percentages based on total alloy weight unless otherwise indicated. “NS” in Table 1 indicates that the particular UNS specification does not specify a concentration for the element. TABLE 1 Type Esshete Nitronic Type Type NF709 201L 1250 60 309S 310S UNS Number none S20100 S21500 S21800 S30908 S31008 Carbon 0.10 max 0.03 max 0.06-0.15 0.10 max 0.08 max 0.08 max Molybdenum 1.0-2.0 NS 0.8-1.2 NS NS NS Chromium 19.0-23.0 16-18 14-16 16-18 22-24 24-26 Nickel 22.0-28.0 3.5-5.5  9-11 8-9 12-15 19-22 Niobium 0.10-0.40 0.75-1.25 0.75-1.25 NS NS NS Manganese 1.50 max 5.5-7.5 5.5-7.5 7-9  2.0 max  2.0 max Silicon NS NS NS 3.5-4.5 0.75 max 0.75 max Titanium 0.02-0.20 NS NS NS NS NS Nitrogen 0.10-0.25 0.25 max NS 0.08-0.18 NS NS

With reference to Table 1, basic AISI Type 201 stainless steel is similar to standard 18 chromium-8 nickel stainless steels, but with a fraction of nickel replaced with manganese to lower alloy cost. In general, Type 201 alloy does not possess sufficient creep and oxidation resistance for use at elevated temperatures. Higher-alloyed materials such as the NITRONIC® family of alloys, Esshete 1250 alloy, and 21-6-9 alloy (UNS S21900), include low nickel levels (about 11 weight percent maximum) and significant manganese levels (5-10 weight percent), and are typically designed for high creep strength and moderate environmental resistance. Commercially available heat-resistant stainless steels such as AISI Types 309S and 310S generally include manganese at levels up to about 2 weight percent. These alloys are somewhat deficient in terms of metallurgical stability, which may be tied to their basic compositions inasmuch as the nickel-to-chromium ratio for these two grades results in the formation of significant amounts of brittle phases at typical use temperatures.

It would be advantageous to provide austenitic stainless steels having improved high temperature creep resistance and/or resistance to corrosive attack in high temperature air and/or in high temperature air containing appreciable levels of water vapor. For example, stainless steels exhibiting substantial corrosion resistance in high temperature air including water vapor could be advantageously employed in, for example, parts of energy generation devices including, for example, gas turbines, steam turbines, and fuel cells, which are subjected to highly corrosive high temperature-high water vapor content environments. Such parts include heat exchangers, recuperators, tubing, pipe, and certain structural parts. Alloys exhibiting substantial corrosion resistance in high temperature air also may be advantageously applied in certain devices for high temperature processing, treatment, or extraction of chemicals or minerals, or for high temperature processing or treatment of alloys. Stainless steels exhibiting both substantial high temperature creep resistance as well as significant corrosion resistance could be advantageously adapted for use in parts of the foregoing devices that are subjected to high stresses.

SUMMARY

According to the present disclosure, austenitic stainless steels are provided having improved high temperature creep resistance and/or improved resistance to corrosion when exposed to a high temperature air environment. As used herein, “high temperature” refers to temperatures in excess of about 100° F. (about 37.8° C.). According to one aspect of the present disclosure, an austenitic stainless steel is provided including: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; greater than 1.5 to 4.0 manganese; 0.20 to 0.75 niobium; up to 0.1 titanium; iron; and incidental impurities. In certain non-limiting embodiments, the manganese content of the steel is at least 1.6 up to 4.0 weight percent. Also, in certain non-limiting embodiments, the austenitic stainless steel further includes one or more of the following elements: greater than 0 to 0.50 silicon; greater than 0 to 0.30 aluminum; greater than 0 to 0.02 sulfur; greater than 0 to 0.05 phosphorus; greater than 0 to 0.1 zirconium; and greater than 0 to 0.1 vanadium. According to certain non-limiting embodiments, the titanium and/or aluminum content of the steel is no greater than 0.1 weight percent.

As used herein, the use of “up to” without reference to a lower limit includes the absence of the referenced element. Also, as used herein, “no greater than” with reference to titanium and aluminum content includes the absence of these elements.

According to another aspect of the present disclosure, an austenitic stainless steel is provided that includes: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; greater than 1.5 to 4.0 manganese; 0.20 to 0.75 niobium; up to 0.1 titanium; up to 0.50 silicon; up to 0.30 aluminum; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium; iron; and incidental impurities. In certain non-limiting embodiments, the manganese content of the steel is at least 1.6 up to 4.0 weight percent. Also, according to certain non-limiting embodiments, the titanium and/or aluminum content of the steel is no greater than 0.1 weight percent.

According to yet another aspect of the present disclosure, an austenitic stainless steel is provided that consists essentially of the following: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; greater than 1.5 to 4.0 manganese; 0.20 to 0.75 niobium; up to 0.1 titanium; up to 0.50 silicon; up to 0.30 aluminum; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium; iron; and incidental impurities. According to certain non-limiting embodiments, the manganese content of the steel is at least 1.6 up to 4.0 weight percent.

According to yet a further aspect of the present disclosure, an austenitic stainless steel is provided that consists of: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; greater than 1.5 to 4.0 manganese; 0.20 to 0.75 niobium; up to 0.1 titanium; up to 0.50 silicon; up to 0.30 aluminum; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium; iron; and incidental impurities. In certain non-limiting embodiments of the steel, the manganese content of the steel is at least 1.6 up to 4.0 weight percent.

Another aspect of the present disclosure is directed to an austenitic stainless steel including, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; up 4.0 manganese; 0.20 to 0.75 niobium; at least one of no greater than 0.1 titanium and no greater than 0.1 aluminum; iron; and incidental impurities.

A further aspect of the present disclosure is directed to an austenitic stainless steel consisting essentially of, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; up to 4.0 manganese; 0.20 to 0.75 niobium; at least one of no greater than 0.1 titanium and no greater than 0.1 aluminum; up to 0.50 silicon; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium; iron; and incidental impurities. In certain non-limiting embodiments, the steel includes at least 1.5 up to 4.0 weight percent manganese, while in other embodiments the steel includes 1.6 up to 4.0 weight percent manganese.

Yet a further aspect of the present disclosure is directed to an austenitic stainless consisting of, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; up to 4.0 manganese; 0.20 to 0.75 niobium; at least one of no greater than 0.1 titanium and no greater than 0.1 aluminum; up to 0.50 silicon; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium; iron; and incidental impurities. In certain non-limiting embodiments, the steel includes at least 1.5 up to 4.0 weight percent manganese, while in other embodiments the steel includes 1.6 up to 4.0 weight percent manganese.

According to yet another aspect of the present invention, an article of manufacture is provided including an austenitic stainless steel having a composition according to the present disclosure. Non-limiting embodiments of the article of manufacture include, for example, energy generation devices and parts of such devices. For example, the article of manufacture may be selected from a gas turbine, a steam turbine, a fuel cell, a heat exchanger, a recuperator, a tube, a pipe, a structural part, and other parts for any of those devices. Other examples of the article of manufacture include equipment or piping, tubing, and other parts for equipment for high temperature processing, treatment, or extraction of chemicals and minerals, or for high temperature processing or treatment of alloys

The reader will appreciate the foregoing details, as well as others, upon considering the following detailed description of certain non-limiting embodiments within the present disclosure. The reader also may comprehend additional advantages and details upon evaluating or using alloys and articles of manufacture within the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the alloys and articles described herein may be better understood by reference to the accompanying drawing in which:

FIG. 1 is a plot of weight change over time for alloy samples exposed at 1300° F. (704° C.) in air containing 10% water vapor;

FIG. 2 is a plot of weight change over time for alloy samples exposed at 1400° F. (760° C.) in air containing 7% water vapor;

FIG. 3 is a plot of weight change over time for alloy samples exposed at 1500° F. (815° C.) in air containing 7% water vapor;

FIGS. 4(a) and 4(b) are micrographs of oxide scale formed on alloy samples exposed to high temperature environments including water vapor;

FIG. 5 is a graph of oxide composition, measured as a molar ratio of MnO to Cr₂O₃, for several alloys subjected to high temperature environments including water vapor;

FIG. 6 is a plot of chromium content of two alloy samples as a function of depth into the sample;

FIG. 7 is a plot of chromium content of two alloy samples as a function of depth into the sample;

FIG. 8 is a graph of oxide composition, measured as a molar ratio of MnO to Cr₂O₃, for high manganese and low manganese samples subjected to high temperature environments including 7% water vapor; and

FIG. 9 is a plot of weight change over time for alloy samples exposed at 1400° F. (760° C.) in air containing 10% water vapor.

DESCRIPTION OF CERTAIN NON-LIMITING EMBODIMENTS

Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, processing conditions and the like used in the present description and claims are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, any numerical parameters set forth in the following description and the attached claims are approximations that may vary depending upon the desired properties one seeks to obtain in the alloys and articles according to the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the present disclosure are approximations, the numerical values set forth in any specific examples herein are reported as precisely as possible. Any numerical values, however, inherently contain certain errors, such as, for example, equipment and/or operator errors, necessarily resulting from the standard deviation found in their respective testing measurements. Also, it should be understood that any numerical range recited herein is intended to include the range boundaries and all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

Any patent, publication, or other disclosure material, in whole or in part, that is said to be incorporated by reference herein is incorporated herein only to the extent that the incorporated material does not conflict with existing definitions, statements, or other disclosure material set forth in this disclosure. As such, and to the extent necessary, the disclosure as set forth herein supersedes any conflicting material incorporated herein by reference. Any material, or portion thereof, that is said to be incorporated by reference herein, but which conflicts with existing definitions, statements, or other disclosure material set forth herein is only incorporated to the extent that no conflict arises between that incorporated material and the existing disclosure material.

As described above, certain austenitic stainless steels have been used in articles and parts exposed to high temperature air or to high temperature air containing significant water vapor. Parts subjected to such conditions include, for example, affected parts of energy generation devices, such as gas turbines, steam turbines, and fuel cells, and heat exchangers and recuperators, and in equipment and parts for high temperature processing, treatment, or extraction of chemicals or minerals, or high temperature processing or treatment of alloys. These steels, however, still suffer from a level of corrosive attack when subjected over time to these conditions. Accordingly, the present inventors undertook to determine whether certain modified austenitic stainless steel chemistries further improved corrosion resistance in high temperature environments. As further described below, the inventors determined that alloys containing 1.5 weight percent or less manganese are subject to oxide scale evaporation and subsequent degradation in air containing water vapor. The inventors' work, in part, focused on certain novel austenitic stainless steel chemistries including more than 1.5 weight percent manganese, along with appreciable levels of chromium and nickel. As a result of their work, the present inventors concluded that an austenitic stainless steel having the broad composition and, more preferably, the nominal composition listed in Table 2 would have substantial resistance to chromium oxide scale evaporation in high temperature air environments and in high temperature air environments including water vapor. The proposed alloy's manganese content is controlled at a minimum level, which was found to significantly improve resistance to high temperature corrosive attack. TABLE 2 Minimum Maximum Nominal Carbon 0.05 0.2 0.10 Nitrogen 0.08 0.2 0.15 Chromium 20 23 20.5 Nickel 25 27 25.5 Molybdenum 1 2 1.5 Manganese greater than 1.5 4.0 1.6 Silicon 0 0.50 0.30 Aluminum 0 0.30 0.25 Sulfur 0 0.02 0.005 Phosphorus 0 0.05 0.03 Niobium 0.20 0.75 0.6 Titanium 0 0.1 — Zirconium 0 0.1 — Vanadium 0 0.1 —

Table 3 provides information on several alloys evaluated during the testing. All heats were melted and subsequently rolled to foil gauge. Heats 1 and 3 were lab heats, heat 2 was prepared as a pilot coil, and heat 4 was a plant heat prepared as a production coil. Heats 1, 3, and 4 were prepared with an aim of 1.0 weight percent manganese, and heat 2 was prepared with an aim of 1.6 weight percent manganese. TABLE 3 Heat 1 Heat 2 Heat 3 Heat 4 Carbon 0.10 0.087 0.076 0.078 Molybdenum 1.54 1.53 1.54 1.50 Chromium 20.01 21.0 20.19 20.4 Nickel 25.42 26.0 25.57 26.0 Niobium 0.65 0.30 0.30 0.34 Manganese 0.99 1.61 1.03 0.99 Titanium 0.077 0.01 — 0.02 Nitrogen 0.143 0.10 0.13 0.1

A comparison of the 1.6 and 1.0 weight percent manganese (nominal) variants listed in Table 3 as heats 2 and 4, respectively, showed that the lower-manganese version is significantly more susceptible to oxide scale evaporation in humidified air, particularly at higher temperatures. This could result in significant environmental attack over time. Testing was conducted as follows.

Samples were exposed in the temperature range 1300-1500° F. (704-815° C.) in wet air. As shown in FIG. 1, both the high manganese sample (approximately 1.6 weight percent manganese, heat 2) and the low manganese sample (approximately 1.0 weight percent manganese, heat 3) exhibited similar oxidation kinetics in terms of weight change (mg/cm²) over time when exposed at 1300° F. (704° C.) in air containing 10% water vapor. The low manganese sample generally exhibited a slightly lower weight gain, with somewhat irregular behavior.

FIG. 2 shows weight change over time for samples of high manganese (heat 2) and the low manganese (heat 4) alloys when the samples were exposed at 1400° F. (760° C.) in air containing 7% water vapor. The samples exhibited significantly different oxidation kinetics under these conditions. The high manganese sample gained weight rapidly during the initial portion of the test, but then the weight gain slowed significantly. After completion of the 5,000 hour test, the two samples exhibited essentially identical weight gain.

FIG. 3 shows weight change over time for samples of high manganese (heat 2) and the low manganese (heat 4) alloys when the samples were exposed at 1500° F. (815° C.) in air containing 7% water vapor. The curve shows that the lower manganese sample exhibited significant oxide scale evaporation during the test period. The higher manganese alloy did not exhibit the same weight change over the limited test exposure.

Samples subjected to 5,000 hours of total exposure under the 1300° F.

(7040) and 1400° F. (760° C.) conditions above were mounted, polished, and examined. The oxide scale that formed on the high manganese samples appeared thin, compact, and essentially featureless. The low manganese variant exhibited subscale void formation after exposure at 1300° F. (704°) in humid air. The oxide scale over these voids, shown in FIG. 4 a, was slightly thicker than the scale elsewhere. Scattered oxide nodules were present on the low manganese samples exposed in humid air at 1400° F. (760° C.). Examples of the nodules are shown in FIG. 4 b. Numerous small “emergent” nodules appeared to be in the process of disrupting the oxide scale.

Samples also were examined under magnification after being exposed to 1500° F. (815° C.) air containing water vapor. It was observed that small nodules of mixed oxides formed in the oxide scale on the low manganese (approximately 1.0 weight percent manganese, heat 4) sample after about 3,000 hours. The low manganese samples was again examined under magnification after about 8,000 hours exposure, and the oxide nodules were found to have grown significantly in size. The high manganese (approximately 1.6 weight percent manganese, heat 2) sample was examined at about 3,500 hours, and no nodules were observed in the oxide scale.

Microanalysis in the scanning electron microscope (SEM) was used to study the general compositional makeup of the oxide scales. The scales were relatively thin (2-3 microns), which made it difficult to extract a detailed compositional profile. Measurements were generally limited to sites near the scale/alloy interface and near the scale/gas interface. It was observed that the high manganese alloy (heat 2) exhibited significantly greater manganese segregation from the alloy to the scale. See FIG. 5, which plots oxide composition, measured as a molar ratio of MnO to Cr₂O₃, as determined using X-ray energy-dispersive spectroscopy (XEDS) in the SEM (semi-quantitative) for several samples at the scale/alloy interface and the scale/gas interface. The low manganese material did not exhibit manganese saturation (i.e., a MnO/Cr₂O₃ ratio of 1.0) at the scale/gas interface at 1300° F. (704° C.) and was borderline saturated at 1400° F. (760° C.). Achieving manganese saturation in the spinel is believed to be important in providing resistance to evaporation.

The same technique (XEDS in the SEM, quantified using standardless and standards-based methods) was used to determine the level and extent of chromium depletion in the underlying metal after exposure to high temperature air including water vapor. FIG. 6 plots the chromium concentration as a function of depth into the sample surface for high manganese and low manganese samples, from heats 2 and 4, respectively, exposed for 5,000 hours at 1300° F. (704° C.) in air containing 10% water vapor. The depletion observed for the low manganese sample is significantly greater in terms of chromium concentration directly adjacent the scale/metal interface Depth of depletion between the samples does not appear to be noticeably different. The chromium profiles derived from each sample appear extremely sharp, indicating that chromium cannot diffuse rapidly from the interior of the sample to the scale/alloy interface.

FIG. 7 is a plot of chromium concentration as a function of depth into the sample surface for high manganese and low manganese samples, heats 2 and 4, respectively, exposed for 5,000 hours at 1400° F. (760° C.) in air containing 7% water vapor. As with FIG. 6, chromium depletion for the low manganese sample was significantly greater than for the high manganese sample at the scale/metal interface. It was observed that the effect of chromium depletion at 1400° F. (760° C.) is not substantially greater in terms of terminal chromium content at the scale/alloy interface relative to what is shown in FIG. 6, but the gradient shown in FIG. 7 runs much deeper into the substrate. This may have resulted because the diffusion of chromium in the metal is rapid enough at 1400° F. (760° C.) to delocalize the effects of chromium depletion due to oxidation.

FIG. 8 is a graph showing oxide composition, measured as a molar ratio of MnO to Cr₂O₃, using XEDS in the SEM (semi-quantitative) for high manganese and low manganese samples, derived from heats 2 and 4, respectively, subjected to high temperature air containing 7% water vapor. The measurements were taken at the scale/alloy interface and the scale/gas interface. The evaluations conducted after exposure to 1300° F. (704° C.) and to 1400° F. (760° C.) air were conducted after about 5,000 hours of exposure time. Those conducted after exposure at 1500° F. (815° C.) were performed after about 3,000 hours of exposure time. The low manganese material did not exhibit manganese saturation (i.e., a MnO/Cr₂O₃ ratio of 1.0) at the scale/gas interface at 1300° F. (704° C.) and at 1500° F. (815° C.), and was borderline saturated at 1400° F. (760° C.).

A set of heats of higher-manganese alloys was prepared to assess how oxidation resistance responds to further increased manganese levels. Table 4 shows the chemical composition of the additional heats, referenced as heats 5 and 6. TABLE 4 Heat 5 Heat 6 Carbon 0.04 0.03 Manganese 2.04 3.82 Phosphorus 0.006 0.006 Sulfur 0.0069 0.003 Silicon 0.26 0.17 Chromium 19.4 19.81 Nickel 23.19 23.22 Aluminum 0.07 0.17 Molybdenum 1.2 1.25 Copper 0.010 0.010 Titanium 0.004 0.004 Nitrogen 0.051 0.058 Niobium 0.39 0.39

FIG. 9 is a plot of sample weight change over time for samples of the alloys of heat 2 (1.61 weight percent manganese), heat 5 (2.04 weight percent manganese), and heat 6 (3.82 weight percent manganese) exposed at 1400° F. (760° C.) in air containing 7% water vapor. The results indicate that higher manganese levels produce higher initial weight gain through oxide scale formation. While the weight gains shown in FIG. 9 did not appear to be problematic, it is believed that higher manganese levels, above about 4 weight percent, would result in further scale formation and weight gains, and the consequent undesirable result of spallation of the material.

Additional heats 7 through 11 in Table 5 were prepared. The heats included less than 0.1 weight percent titanium. Heats 7, 8 and 11 also included less than 0.1 weight percent aluminum. TABLE 5 Heat 7 Heat 8 Heat 9 Heat 10 Heat 11 Carbon 0.086 0.088 0.078 0.091 0.080 Molybdenum 1.54 1.52 1.50 1.52 1.54 Chromium 20.99 20.95 20.4 20.35 25.83 Nickel 25.92 26.02 26.0 25.7 20.42 Niobium 0.30 0.30 0.34 0.38 0.36 Manganese 1.61 1.79 0.99 1.03 1.52 Titanium 0.010 <0.01 0.02 0.001 0.06 Nitrogen 0.0955 0.1130 0.10 0.104 0.12 Silicon 0.41 0.40 0.47 0.33 0.36 Sulfur <0.01 <0.01 0.0001 0.0001 0.0005 Aluminum <0.01 <0.01 0.16 0.34 0.02 Boron 0.0033 0.0029 0.0047 0.0047 0.0052

As discussed above, austenitic stainless steels subjected to stress at high temperature for prolonged periods can be subject to creep. Most austenitic stainless steels include relatively minor levels of titanium and aluminum to facilitate deoxidation of the molten metal during melting and casting. These elements also are precipitated as nitrides and, possibly, intermetallic phases in the solid state. These precipitated phases are very difficult or impractical to dissolve during processing. Excessive nitride formation will have the effect of reducing the level of nitrogen in solid solution, which will reduce the creep strength of the alloy. Nitrides and intermetallic phases also can make processing more difficult, particularly when the steel is formed by being folded or stamped into part shapes.

Accordingly, in order to improve creep strength and the formability of the alloy during folding, stamping, and similar mechanical processing steps, a preferred chemistry for the austenitic stainless steels of the present disclosure includes at least one of no greater than 0.1 weight percent titanium and no greater than 0.1 weight percent aluminum. More preferably, to better enhance creep resistance and formability, the austenitic stainless steels of the present disclosure includes no greater than 0.1 weight percent titanium and no greater than 0.1 weight percent aluminum.

Based on the above, an austenitic stainless having the investigated chemistries and including manganese at levels greater than 1.5 weight percent and up to about 4 weight percent should exhibit advantageous resistance to high temperature attack in air, which may include significant water vapor, and without suffering from excessive scale formation and spallation. More specifically, the broad and nominal alloy compositions shown in Table 2 are proposed as austenitic stainless steels with substantial resistance to corrosive attack in high temperature air and in high temperature air including water vapor. A preferred manganese level is at least 1.6 up to about 4 weight percent, and a more preferred manganese level is at least 1.6 up to about 2.0 weight percent manganese.

An additional proposed alloy chemistry having improved creep resistance and improved formability has the general chemistry shown in Table 2, but includes no greater than 0.1 weight percent titanium and/or no greater than 0.1 weight percent aluminum. The expected improvement in creep resistance resulting from the limits on titanium and/or aluminum content is not necessarily tied to the improved high temperature corrosion resistance provided by controlling the manganese content to the range of greater than 1.5 weight percent up to about 4 weight percent. Instead, the manganese content of the alloy proposed herein having improved creep resistance and formability may be any level up to about 4.0 weight percent. Accordingly, the alloy in the following Table 6 should exhibit advantageous creep resistance and formability properties, and a preferred chemistry includes no greater than 0.1 weight percent titanium and no greater than 0.1 weight percent aluminum. TABLE 6 Minimum Maximum Carbon 0.05 0.2 Nitrogen 0.08 0.2 Chromium 20 23 Nickel 25 27 Molybdenum 1 2 Manganese 0 4.0 Silicon 0 0.50 Aluminum* 0 0.30 Sulfur 0 0.02 Phosphorus 0 0.05 Niobium 0.20 0.75 Titanium* 0 0.1 Zirconium 0 0.1 Vanadium 0 0.1 *At least one of Ti and Al is no greater than 0.1.

An alloy exhibiting advantageous high temperature creep resistance, improved formability, and advantageous resistance to corrosive attack in high temperature air including water vapor would have the composition shown in Table 6 and wherein the composition is further controlled such that the manganese content is greater then 1.5 up to about 4.0 weight percent, preferably is at least 1.6 up to about 4.0 weight percent, and more preferably is at least 1.6 up to about 2.0 weight percent. Such an alloy could be advantageously applied in making, for example, structural parts and other parts of the previously mentioned energy generation devices and processing, treatment, or extraction devices that are both subjected to stress and exposed to high temperature air including water vapor.

An optional limitation on the austenitic stainless steels chemistries proposed herein, established to better ensure substantial resistance to creep, is that the niobium to carbon ratio in the alloys satisfies the following formula: 0.7<0.13(niobium/carbon)≦1.0, wherein the niobium and carbon contents in the formula are expressed in atom percentages.

Heats of the novel corrosion resistant austenitic stainless steels disclosed herein may be made by conventional means, such as by the conventional technique of vacuum melting scrap and other feed materials. The resulting heats may be processed by conventional techniques into billets, slabs, plates, coils, sheets, and other intermediate articles, and then further processed into final articles of manufacture. The enhanced formability of embodiments of alloys within the present disclosure including no greater than 0.1 weight percent of titanium and/or no greater than 0.1 weight percent aluminum allows flat mill products (such as strip, sheet, plate, coil, and the like) formed from the alloys to be further processed into articles having relatively complicated shapes. This characteristic of the alloys is an advantage relative to NF709 alloy, which has more limited formability and has commonly only been processed by extrusion into seamless pipe.

The novel austenitic stainless steels according to the present disclosure may be used in any suitable application and environment, but the alloys are particularly suited for use in equipment and parts subjected for extended periods to high temperature, or to both high temperature and significant water vapor. For example, the creep resistance and/or high temperature corrosion resistance of the alloys disclosed herein makes them particularly suitable for use in: tubing, piping, structural parts, and other parts of equipment adapted for high temperature processing, treatment, or extraction of chemicals or minerals, or high temperature processing or treatment of alloys; tubing, piping, structural parts, and other parts of energy generation devices such as, for example, gas turbines, steam turbines, and fuel cells; and parts of heat exchangers, recuperators, and other equipment handling gas streams used or generated by energy generation devices. Other applications for the alloys disclosed herein will be apparent to those of ordinary skill upon considering the present description of the alloys.

Although the foregoing description has necessarily presented a limited number of embodiments of the invention, those of ordinary skill in the relevant art will appreciate that various changes in the compositions and other details of the examples that have been described and illustrated herein in order to explain the nature of the invention may be made by those skilled in the art, and all such modifications will remain within the principle and scope of the invention as expressed herein and in the appended claims. It will also be appreciated by those skilled in the art that changes could be made to the embodiments above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications that are within the principle and scope of the invention, as defined by the claims. 

1. An austenitic stainless steel comprising, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; to 27 nickel; 1 to 2 molybdenum; greater than 1.5 to 4.0 manganese; 0.20 to 0.75 niobium; up to 0.1 titanium; iron; and incidental impurities.
 2. The austenitic stainless steel of claim 1, wherein the steel comprises 1.6 to 4.0 weight percent manganese, based on total weight of the steel.
 3. The austenitic stainless steel of claim 1, further comprising at least one of the following, in weight percentages based on total weight of the steel: greater than 0 to 0.50 silicon; greater than 0 to 0.30 aluminum; greater than 0 to 0.02 sulfur; greater than 0 to 0.05 phosphorus; greater than 0 to 0.1 zirconium; and greater than 0 to 0.1 vanadium.
 4. The austenitic stainless steel of claim 1, wherein at least one of titanium and aluminum is limited to no greater than 0.1 weight percent, based on total weight of the steel.
 5. The austenitic stainless steel of claim 1, wherein the ratio of niobium to carbon in the steel satisfies the formula 0.7<0.13 (niobium/carbon)≦1.0, wherein the niobium and carbon contents in the formula are expressed in atom percentages.
 6. The austenitic stainless steel of claim 1, comprising, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; greater than 1.5 to 4.0 manganese; 0.20 to 0.75 niobium; up to 0.1 titanium; up to 0.50 silicon; up to 0.30 aluminum; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium; iron; and incidental impurities.
 7. The austenitic stainless steel of claim 6, wherein the steel comprises 1.6 to 4.0 weight percent manganese, based on total weight of the steel.
 8. The austenitic stainless steel of claim 6, wherein at least one of titanium and aluminum is limited to no greater than 0.1 weight percent, based on total weight of the steel.
 9. The austenitic stainless steel of claim 1, wherein the steel consists essentially of, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; greater than 1.5 to 4.0 manganese; 0.20 to 0.75 niobium; up to 0.1 titanium; up to 0.50 silicon; up to 0.30 aluminum; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium; iron; and incidental impurities.
 10. The austenitic stainless steel of claim 9, wherein the steel consists essentially of, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; 1.6 to 4.0 manganese; 0.20 to 0.75 niobium; up to 0.1 titanium; up to 0.50 silicon; up to 0.30 aluminum; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium; iron; and incidental impurities.
 11. The austenitic stainless steel of claim 10, wherein at least one of titanium and aluminum is limited to no greater than 0.1 weight percent, based on total weight of the steel.
 12. The austenitic stainless steel of claim 1, wherein the steel consists of, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; greater than 1.5 to 4.0 manganese; 0.20 to 0.75 niobium; up to 0.1 titanium; up to 0.50 silicon; up to 0.30 aluminum; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium; iron; and incidental impurities.
 13. The austenitic stainless steel of claim 12, wherein the steel consists of, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; 1.6 to 4.0 manganese; 0.20 to 0.75 niobium; up to 0.1 titanium; up to 0.50 silicon; up to 0.30 aluminum; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium; iron; and incidental impurities.
 14. The austenitic stainless steel of claim 12, wherein at least one of titanium and aluminum is limited to no greater than 0.1 weight percent, based on total weight of the steel.
 15. An article of manufacture including an austenitic stainless steel comprising, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; greater than 1.5 to 4.0 manganese; 0.20 to 0.75 niobium; up to 0.1 titanium; iron; and incidental impurities.
 16. The article of manufacture of claim 15, wherein the austenitic stainless comprises 1.6 to 4.0 weight percent manganese, based on total weight of the steel.
 17. The article of manufacture of claim 15, wherein the austenitic stainless further comprises at least one of the following, in weight percentages based on total weight of the steel: greater than 0 to 0.50 silicon; greater than 0 to 0.30 aluminum; greater than 0 to 0.02 sulfur; greater than 0 to 0.05 phosphorus; greater than 0 to 0.1 zirconium; and greater than 0 to 0.1 vanadium.
 18. The article of manufacture, wherein at least one of titanium and aluminum in the austenitic stainless steel is limited to no greater than 0.1 weight percent, based on total weight of the steel.
 19. The article of manufacture of claim 15, wherein the article of manufacture is one of an energy generation device and a device for processing or treating at least one of a chemical, a mineral, or an alloy.
 20. The article of manufacture of claim 15, wherein the article of manufacture is selected from the group consisting of a gas turbine, a steam turbine, a fuel cell, and parts for any of those articles.
 21. The article of manufacture of claim 15, wherein the article of manufacture is a device or part that receives gases used or generated by an energy generation device.
 22. The article of manufacture of claim 15, wherein the article of manufacture is one of a heat exchanger, a heat exchanger part, a recuperator, and recuperator part.
 23. The article of manufacture of claim 15, wherein the article of manufacture is a part of a device adapted for at least one of: processing at high temperature at least one of a chemical, a mineral, and an alloy; treating at high temperature at least one of a chemical, a mineral, and an alloy; or extracting at high temperature at least one of a chemical and a mineral.
 24. An austenitic stainless steel comprising, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; to 27 nickel; 1 to 2 molybdenum; up 4.0 manganese; 0.20 to 0.75 niobium; at least one of no greater than 0.1 titanium and no greater than 0.1 aluminum; iron; and incidental impurities.
 25. The austenitic stainless steel of claim 24, wherein the steel comprises 1.6 to 4.0 weight percent manganese, based on total weight of the steel.
 26. The austenitic stainless steel of claim 24, further comprising at least one of the following, in weight percentages based on total weight of the steel: greater than 0 to 0.50 silicon; greater than 0 to 0.02 sulfur; greater than 0 to 0.05 phosphorus; greater than 0 to 0.1 zirconium; and greater than 0 to 0.1 vanadium.
 27. The austenitic stainless steel of claim 24, wherein the ratio of niobium to carbon in the steel satisfies the formula 0.7<0.13 (niobium/carbon)≦1.0, wherein the niobium and carbon contents in the formula are expressed in atom percentages.
 28. The austenitic stainless steel of claim 24, wherein the steel consists essentially of, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; up to 4.0 manganese; 0.20 to 0.75 niobium; at least one of no greater than 0.1 titanium and no greater than 0.1 aluminum; up to 0.50 silicon; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium; iron; and incidental impurities.
 29. The austenitic stainless steel of claim 24, wherein the steel consists essentially of, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; 1.6 to 4.0 manganese; 0.20 to 0.75 niobium; at least one of no greater than 0.1 titanium and no greater than 0.1 aluminum; up to 0.50 silicon; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium; iron; and incidental impurities.
 30. The austenitic stainless steel of claim 24, wherein the steel consists of, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; up to 4.0 manganese; 0.20 to 0.75 niobium; at least one of no greater than 0.1 titanium and no greater than 0.1 aluminum; up to 0.50 silicon; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium; iron; and incidental impurities.
 31. The austenitic stainless steel of claim 30, wherein the steel consists of, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; 1.6 to 4.0 manganese; 0.20 to 0.75 niobium; at least one of no greater than 0.1 titanium and no greater than 0.1 aluminum; up to 0.50 silicon; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium; iron; and incidental impurities.
 32. An article of manufacture including an austenitic stainless steel comprising, in weight percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; up to 4.0 manganese; 0.20 to 0.75 niobium; at least one of no greater than 0.1 titanium and no greater than 0.1 aluminum; iron; and incidental impurities.
 33. The article of manufacture of claim 32, wherein the austenitic stainless comprises 1.6 to 4.0 weight percent manganese, based on total weight of the steel.
 34. The article of manufacture of claim 32, wherein the austenitic stainless further comprises at least one of the following, in weight percentages based on total weight of the steel: greater than 0 to 0.50 silicon; greater than 0 to 0.02 sulfur; greater than 0 to 0.05 phosphorus; greater than 0 to 0.1 zirconium; and greater than 0 to 0.1 vanadium.
 35. The article of manufacture of claim 32, wherein the article of manufacture is one of an energy generation device and a device for processing or treating at least one of a chemical, a mineral, or an alloy.
 36. The article of manufacture of claim 32, wherein the article of manufacture is selected from the group consisting of a gas turbine, a steam turbine, a fuel cell, and parts for any of those articles.
 37. The article of manufacture of claim 32, wherein the article of manufacture is a device or part that receives gas streams used or generated by an energy generation device.
 38. The article of manufacture of claim 37, wherein the article of manufacture is one of a heat exchanger, a heat exchanger part, a recuperator, and a recuperator part.
 39. The article of manufacture of claim 32, wherein the article of manufacture is a device or part of a device adapted for at least one of: processing at high temperature at least one of a chemical, a mineral, and an alloy; treating at high temperature at least one of a chemical, a mineral, and an alloy; or extracting at high temperature at least one of a chemical and a mineral. 